From published European Patent document EP-PS 146 716, it is known how to produce the load-bearing structure of a vehicle bodywork by using nodal connector elements for joining together hollow section bearers. Extruded aluminum sections are used as the hollow section bearers and cast aluminum joint pieces are used as the nodal connector elements. Such a bodywork design offers improved corrosion resistance and weight savings.
It is also known from the prior art how to construct a hollow-section longitudinal bearer assembly of a vehicle frame from a plurality of longitudinal bearer members. These individual longitudinal bearer members are joined together, in end to end fashion, by a surrounding weld seam beginning with the forward most bearer member and working rearwards. Each successive longitudinal bearer member beginning with the forwardmost bearer member is dimensioned with a progressingly greater wall thickness and/or is made of a progressively higher grade of material than its preceding bearer member such that the resulting longitudinal bearer assembly has a graduated stiffness or resistance to deformation which increases toward the middle of the vehicle. Thus, in the event of a minor accident, usually only one or two of the forwardmost bearer members will experience deformation without damage to the adjacent rearwardly disposed bearer members. This arrangement greatly simplifies the repair of the vehicle front and also avoids an undesirable total deformation of the vehicle frame in less severe accidents.
In the vehicle frame of the type described, the repair of a deformed longitudinal bearer assembly is accomplished by cutting out the damaged bearer members and welding on new bearer members. For the case where the longitudinal bearer assembly comprises sheet metal stampings, the described repair procedure does not give rise to any major structural problems since the original strength of sheet metal is substantially retained even after welding.
European Patent document 0146 716 discloses a positive-locking connection scheme in a longitudinal bearer assembly in a vehicle body wherein an aluminum bearer, formed as an extruded section, is inserted into a tubular aluminum recess or socket formed as a cast piece. The aluminum bearer is then welded to the front edge of this tubular recess by a surrounding weld seam. Such a connection is stable and economical.
However, it is well known that the thermal welding of heat treatable aluminum alloys produces a significant decrease in the strength in the material in the so-called "zone of thermal influence" immediately adjacent the weld seam. A surrounding weld connection, such as is used in the above-described bearer joint, is especially critical, since it produces a weakening in a continuous transverse plane. The decrease in the strength of the bearer joint caused by the welding procedure during the initial assembly can be factored into the dimensions of the component parts. However, in view of the repeat welding procedure required each time a damaged bearer is cut out and replaced by a new bearer, an uncontrollable amount of damage to the grain texture of the tubular recess of the connector results. This gives rise to a large decrease in the original strength of the material in the heat affected zone and a danger of fracture of the bearer joint during heavy load conditions.
This is particularly problematic in the situation where the bearer member is part of a segmented longitudinal bearer assembly, since in this situation, it is taken for granted that the less stiff and more easily deformable forward bearer members of the longitudinal bearer assembly will necessarily be replaced during repairs after an accident. Further, the forward bearer members may even need to be replaced one or more times for normal wear or fatigue during the lifetime of the vehicle. The welding procedures repeatedly required in this case would unacceptably weaken the joint by progressive structural change in the zone of thermal influence of the material of the aluminum alloy.
While it is conceivable to incorporate an acceptable safety factor into the initial design of the bearer joint such that the wall thicknesses of the component parts are sufficiently large to compensate for the aforesaid severe decrease in strength caused by successive welding procedures, this is not a practical solution in view of the additional cost and weight involved. Furthermore, it is desirable to provide the customer with a vehicle body which retains its original strength even after repairs.
German patent document DE 3 740 402 discloses another design for a front longitudinal bearer assembly which comprises two longitudinal bearer members disposed forwardly of the front suspension mounting.
Here too, the forwardmost longitudinal bearer member is designed as a deformation element such that in the event of a minor collision, it will deform more easily than the adjacent rearwardly disposed longitudinal bearer member. The longitudinal bearer members are arranged in succession and are connected together at abutting ends or bearing surfaces. The abutting bearing surfaces are designed as rotational surfaces in the transverse direction. A central, axially aligned bolt connection is used to join the abutting bearing surfaces of the two longitudinal-member portions.
This configuration is especially advantageous when the longitudinal bearer members are formed as extruded aluminum sections, since the easily accessible bolt connection permits quick and simple replacement of a deformed forward longitudinal bearer member after a minor collision. Also, the strength loss problems associated with the replacement of aluminum welds is not present here. However, the cost of the bolt connections used in this design are relatively high.